1. Field of Invention
The invention relates to fluid pressure control via control over pump speed. More specifically, the invention embodies a pressure controller that controls pump speed based upon inlet and outlet fluid pressure, wherein pump speed is adjusted to maintain both inlet and outlet fluid pressures within specified ranges.
2. Description of Related Art
It is often desirable and most effective to use a pump to move fluids over a significant distance through a channel from one place to another. Pumps are common structures and appear in all sorts of configurations, can operate on all sorts of different principles, can pump all sorts of different fluids, and can pump those fluids over a wide range of distances and terrain. Generally some type of drive mechanism actuates the pump that moves the fluid.
The use of pumps may be the only practical way that fluid can be moved from one place to another over an intervening elevation. In particular, where the origin and destination of the fluid are separated either by terrain having multiple changes in elevation or by a sufficiently great distance, it may be necessary to utilize multiple pumps serially positioned along the fluid channel in order to maintain fluid flow, especially if flow is to be maintained at a relatively constant rate. Another situation in which the problem of unsteady fluid flow may best be solved using a series of pumps positioned at various intervals along the fluid channel is where the fluid source from which fluid arrives at the inlet of the first pump does not provide a steady fluid volume or fluid pressure. Such pumping of fluids over significant distances and varied terrain at a steady flow rate is often a desire in military applications. In particular, the military is primarily interested in transporting two different fluids, water, which is used to cook, clean, or drink, and petroleum based fuel, which is used to power vehicles and machines.
The above mentioned multiple pump, steady-flow solution suggests a pattern of sections of fluid channel in between serially displaced pumps. Such a pattern is often referred to as a “daisy chain.” Such a daisy chain of pumps may consist of, for instance, a series of pumps for which each outlet is connected to a hose, the opposite end of which connects to the inlet of another pump, except that the outlet of the last pump in the series is not further connected to another pump, but discharges the pumped fluid to a desired location. For such a daisy chain of pumps to operate properly, each pump must maintain a certain level of flow through the segment of pipe into which it pumps fluid so that the flow reaching the next pump is sufficient for that next pump to maintain the flow in the channel section that follows it, and so on.
In at least one application of such daisy chain, the daisy chain can be designed (particularly through the specification of the horizontal distance between pumps) such that identical pumps can be driven at identical speeds to produce identical inlet and outlet pressures at each pump, and a constant flow rate from the first pump to the last. Such a situation is highly idealized, but is the situation often desired by the military, which operates under conditions that necessitate standardization.
The U.S. military has, in fact, provided a performance specification and a test procedure for a pump that may be used in just such a daisy chain. The specification, MIL-PRF-53051, is entitled “Performance Specification: Pumping Assemblies, Wheel-Mounted, Diesel Engine Driven, Bulk Transfer, Fuel and Potable Water Pumping Service,” Rev. B (Aug. 6, 1998). The test procedure, MKP-1398-ATP, is entitled “350 gpm Pump Acceptance Test Procedure,” Rev. E (Jan. 17, 2001). Both of these documents, MIL-PRF-53051 and MKP1398ATP, are herein incorporated in their entirety by specific reference.
Considering now an individual pump having an inlet and outlet for fluid, where the inlet and outlet are connected to a fluid flow channel that is part of a larger fluid flow system (e.g., a daisy chain), if all other variables within the fluid flow system that impact inlet and outlet pressure (“fluid flow system parameters”) remain constant, the speed at which the pump is driven will be directly related to outlet pressure and inversely related to inlet pressure. That is, with all other variables constant, as the speed of the pump increases, the fluid pressure at the outlet increases and the fluid pressure at the inlet decreases. The exact nature of these relationships (e.g., whether first order, second order, exponential, or other) depends on the characteristics of the pump and the fluid flow system parameters.
From the existence of these separate relationships of the inlet and outlet pressures to the pump speed, it becomes evident that the pump speed can be made an independent variable used to adjust the inlet and outlet pressures. Where the fluid flow system parameters permit, pump speed can be used to maintain the inlet and outlet pressures within a separately prescribed range. That is, adjustment of the pump speed can keep the inlet pressure and the outlet pressure from dropping below an individually set minimum value and from raising above an individually set maximum value. The ability to control inlet and outlet pressures through adjustments in pump speed is maintained so long as the configuration of the fluid flow system has not caused excessive pressures to build up at either the inlet or the outlet, and so long as sufficient fluid is provided to the inlet such that a minimum pressure is there maintained.
Examples of how pump speed can be used to control inlet and outlet pressures follow. Where the outlet pressure is measured to be above a preferred maximum value (the maximum outlet pressure limit), the pump speed can be reduced so that the outlet pressure is consequently reduced. As another example, where the inlet pressure is measured to be above a preferred maximum value (the maximum inlet pressure limit), the pump speed can be increased so that the inlet pressure is consequently reduced.
A limitation to pressure control through pump speed is provided by the minimum and maximum speed of the pump drive mechanism. Where the drive mechanism is an internal combustion engine, the minimum speed will be provided by the idle speed of the engine. The maximum speed of such an engine is a function of engine design, and will likely be specified by the engine manufacturer as a safe maximum continuous operating speed.
Besides the relationship wherein the inlet and outlet pressures vary according to the pump speed, there is, at any given pump speed (as long as there is sufficient flow in the fluid flow system), a direct relationship between the inlet pressure and the outlet pressure. That is, at a given pump speed, as the inlet pressure increases, the outlet pressure also increases, and as the inlet pressure decreases, the outlet pressure also decreases. Because the action of the pump is directional (in the direction of fluid flow) the reverse of this relationship is not true; while outlet pressure is dependent upon inlet pressure, inlet pressure is not dependent upon outlet pressure.
Because the inlet and outlet pressures may be independently affected by the fluid flow system parameters, and are not solely affected by the pump speed, it is necessary in order to allow rational control over inlet and outlet pressure through pump speed that the fluid flow system is carefully designed not to generate conditions outside the acceptable ranges when the pump is operating normally. If the fluid flow system is not so designed a situation could arise where the measured output pressure was above the maximum outlet pressure limit, indicating a need to decrease pump speed, and the measured inlet pressure was above the maximum inlet pressure limit, indicating a need to increase pump speed. Such a condition is not amenable to rational control that maintains both inlet and outlet pressures within specified ranges.
Another variable that will affect the inlet and outlet pressure is the particular fluid being pumped. With all other fluid flow system parameters constant, the difference between the inlet and outlet pressure will be greater for a fluid of higher specific weight. So, for any given pump speed and inlet pressure, the outlet pressure will be higher when a fluid of relatively high specific weight is being pumped than will be the outlet pressure when a fluid of comparatively lower specific weight is being pumped. Thus, a pump speed calibration correlating inlet and outlet pressures with particular pump speeds will necessarily be different for fluids of varying specific weight.
Existing pressure controllers for maintaining inlet and outlet fluid pressures at a pump have used analog electrical circuitry incorporating PID (proportional-integral-derivative) control in a feedback loop. A disadvantage of analog control, generally, is a lack of precision. A disadvantage of PID control, generally, is the complex circuitry required. For a pump designed to pump multiple fluids, a particularly significant disadvantage to analog-PID pressure control is its inability to easily control pressures for multiple fluids because each fluid may require the implementation of unique pressure limits and certainly will require the utilization of different pump speed calibration curves. Specifically, under conditions where the pump drive mechanism normal operating speed is specified, because of the varying relationship between inlet and outlet pressure among fluids of varying specific weight, the specified pressure limits for two such fluids must be different. A possible but inconvenient way to provide stable analog-PID pressure control for multiple fluids is to provide separate circuits correlated with each fluid potentially pumped. Generally, for existing systems of multiple analog-PID circuits, the proper circuit for a given application must be manually selected.